Network Working Group L. Nguyen
Request for Comments: 4812 A. Roy
Category: Informational Cisco Systems
A. Zinin
Alcatel-Lucent
March 2007
OSPF Restart Signaling
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
OSPF is a link-state intra-domain routing protocol used in IP
networks. Routers find new and detect unreachable neighbors via the
Hello subprotocol. Hello OSPF packets are also used to ensure two-
way connectivity within time. When a router restarts its OSPF
software, it may not know its neighbors. If such a router sends a
Hello packet on an interface, its neighbors are going to reset the
adjacency, which may not be desirable in certain conditions.
This memo describes a vendor-specific mechanism that allows OSPF
routers to inform their neighbors about the restart process. Note
that this mechanism requires support from neighboring routers. The
mechanism described in this document was proposed before Graceful
OSPF Restart, as described in RFC 3623, came into existence. It is
implemented/supported by at least one major vendor and is currently
deployed in the field. The purpose of this document is to capture
the details of this mechanism for public use. This mechanism is not
an IETF standard.
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RFC 4812 OSPF Restart Signaling March 2007
Table of Contents
1. Introduction ....................................................22. Proposed Solution ...............................................22.1. Sending Hello Packets with the RS-bit Set ..................32.2. Receiving Hello Packets with the RS-Bit Set ................32.3. Ensuring Topology Stability ................................43. Backward Compatibility ..........................................44. Security Considerations .........................................45. IANA Considerations .............................................46. References ......................................................56.1. Normative References .......................................56.2. Informative References .....................................5Appendix A. Acknowledgements ......................................61. Introduction
While performing a graceful restart of OSPF software [RFC3623],
routers need to prevent their neighbors from resetting their
adjacencies. However, after a reload, routers may not be aware of
the neighbors they had adjacencies with in their previous
incarnations. If such a router sends a Hello packet on an interface
and this packet does not list some neighbors, those neighbors will
reset the adjacency with the restarting router.
This document describes a technique that allows restarting routers to
inform their neighbors that they may not know about some neighbors
yet and the absence of some router IDs in the Hello packets should be
ignored.
2. Proposed Solution
With this Restart Signaling Solution, a new bit, called RS (restart
signal), is introduced into the Extended Options (EO) TLV in the
Link-Local Signaling (LLS) block (see [RFC4813]). The value of this
bit is 0x00000002; see Figure 1 below.
+---+---+---+---+---+---+---+- -+---+---+---+---+---+---+---+---+
| * | * | * | * | * | * | * |...| * | * | * | * | * | * | RS| LR|
+---+---+---+---+---+---+---+- -+---+---+---+---+---+---+---+---+
Figure 1. Bits in Extended Options TLV
For a definition of the LR-bit, see [RFC4811].
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RFC 4812 OSPF Restart Signaling March 20072.1. Sending Hello Packets with the RS-bit Set
OSPF routers should set the RS-bit in the EO-TLV attached to a Hello
packet when it is not known that all neighbors are listed in this
packet, but the restarting router wants them to preserve their
adjacencies. The RS-bit must not be set in Hello packets longer than
RouterDeadInterval seconds.
2.2. Receiving Hello Packets with the RS-Bit Set
When an OSPF router receives a Hello packet containing the LLS block
with the EO-TLV that has the RS-bit set, the router should skip the
two-way connectivity check with the announcing neighbor (i.e., the
router should not generate a 1-WayReceived event for the neighbor if
it does not find its own router ID in the list of neighbors as
described in Section 10.5 of [RFC2328]), provided that the neighbor
Finite State Machine (FSM) for this neighbor is in the Full state.
The router should also send a unicast Hello back to the sender in
reply to a Hello packet with RS-bit set. This is to speed up
learning of previously known neighbors. When sending such a reply
packet, care must be taken to ensure that the RS-bit is clear in it.
Two additional fields are introduced in the neighbor data structure:
RestartState flag and ResyncTimeout timer. RestartState flag
indicates that a Hello packet with the RS-bit set has been received
and the local router expects its neighbor to go through the Link
State Database (LSDB) resynchronization procedure using [RFC4811].
ResyncTimeout is a single-shot timer limiting the delay between the
first seen Hello packet with the RS-bit set and initialization of the
LSDB resynchronization procedure. The length of ResyncTimeout timer
is RouterDeadInterval seconds.
When a Hello packet with the RS-bit set is received and RestartState
flag is not set for the neighbor, the router sets RestartState flag
and starts ResyncTimeout timer. If ResyncTimeout expires,
RestartState flag is cleared and a 1-WayReceived event is generated
for the neighbor. If, while ResyncTimeout timer is running, the
neighbor starts LSDB resynchronization procedure using [RFC4811],
ResyncTimeout timer is canceled. The router also clears RestartState
flag on completion of the LSDB resynchronization process.
Two or more routers on the same segment cannot have Hello packets
with the RS-bit set at the same time, as can be the case when two or
more routers restart at about the same time. In such a scenario, the
routers should clear the RestartState flag, cancel the ResyncTimeout
timer, and generate a 1-WayReceived event.
Nguyen, et al. Experimental [Page 3]

RFC 4812 OSPF Restart Signaling March 20072.3. Ensuring Topology Stability
Under certain circumstances, it might be desirable to stop announcing
the restarting router as fully adjacent if this may lead to possible
routing loops. In order to provide this functionality, a
configurable option is provided on the neighboring routers that
instructs the OSPF process to follow the logics described below.
When an OSPF router schedules a routing table calculation due to a
change in the contents of its LSDB, it should also reset all
adjacencies with restarting routers (those with RestartState set to
TRUE) by clearing the RestartState neighbor flags, canceling
ResyncTimeout timers (if running), and generating the 1-WayReceived
events for the neighbor FSMs.
3. Backward Compatibility
The described technique requires cooperation from neighboring
routers. However, if neighbors do not support this technique, they
will just reset the adjacency.
4. Security Considerations
The described technique does not introduce any new security issues
into the OSPF protocol.
5. IANA Considerations
Please refer to the "IANA Considerations" section of [RFC4813] for
more information on the Extended Options bit definitions.
Nguyen, et al. Experimental [Page 4]

RFC 4812 OSPF Restart Signaling March 2007
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